Studies of combustion of a fluid discharged into a combustion chamber have shown that it is important to maintain an internal recirculation of the products of combustion and to fix the position of the flame for most effective stabilization of the combustion process. These desiderata can be achieved at least in part with the system described in the above-identified application. It is, consequently, possible to reduce the excess of air required for full combustion and to provide a Bacharach index in the combustion gas of an acceptable level.
As described in the aforementioned application, one particularly effective technique for assuring intensive internal recirculation of the combustion gases at the burner conjointly with an anchoring of the flame in the combustion chamber resides in introducing the gaseous comburant into this chamber with a helicoidal movement about the axis along which the combustible (fuel) is injected coaxially.
This vortex discharge, known as "swirl" produced a toroidal vortex which, in turn, induces a strong internal recirculation of a comburant gas and combustible mixture and the products of combustion, thereby increasing the possibility of their interaction, i.e. complete combustion, and assuring a vigorous mixture of the components.
As the intensity of the swirl increases, i.e. the kinetic moment of rotation increases in significance with respect to the axial mass flow, the more the toroidal vortex extends in the direction of the injection nozzle if the latter is disposed along the axis of generation of the swirl. As this vortex carries the particles of the combustible which have been incompletely burned, e.g. of soot or, in the case of a combustible liquid, of fuel oil, the latter tend to a deposit on the injection nozzle and accumulate thereon little by little, eventually obstructing the nozzle.
To overcome this disadvantage, it has already been proposed in the aforementioned application to provide an annular chamber around the nozzle which opens substantially at the discharge end of the nozzle and to feed this chamber with a portion of the mass of comburant gas supplied by the blower of the burner.
This system has been found to be most efficacious in preventing the accumulation of deposits upon the combustion nozzle using the flow of comburant extending axially along an annular zone concentric to the nozzle. However, it has been found that the toroidal vortex created by an intense swirl "rises" up to the neck of the burner and flows along the outer surface of the annular chamber, depositing particles of the combustible upon the walls thereof. These deposited particles are transformed into carbon and tarry deposits which encrust these walls.
Such particles of the combustible are capable of being accumulated on all surfaces which are encountered by them and hence all such surfaces tend to be similarly encrusted. The encrustation appears to be independent of the surface area or size of the surface encountered by the particles. For example, particles of the combustible are able to accumulate on the surface of the end of the wall of the tubular chamber which surrounds the nozzle in spite of the fact that it has a thickness of only about 1 mm.
This "rise" or outward movement of a strong swirl discharge arises at one or more points at which the local axial speed inverts with respect to the mean flow (so-called separation points). As the number of such points increases, there can be a sort of rupture of the "vortex", hereinafter referred to as vortex breakdown.